Deciding when to separate the kittens form their mother and litter mates is a very hard decision to make. They are so many things to consider and I would not recommend it until 10 weeks I also would not recommend waiting beyond 12 weeks. Somewhere in that time frame I would separate them.
Separating the kittens from mom and siblings too soon can lead to many risk for the kittens development such as, kneading when they sink their claws in and out of your lap or furniture,(They kneed because they kneed moms stomach when they nurse for both comfort and to help bring the milk down.) False nursing, (which is pretending to nurse your arm or other objects), separation anxiety (where they scream until you come home) and just screaming until you feed them play with them etc. Leaving them with mom too long also causes risk to their development they may suddenly decide to have nothing but moms milk even if they were eating the canned food you gave them yesterday. Leaving them with mom too long also puts mom at risk for health problems.
A cat nursing too long may soon look ran-down and uncared for not mater how much of the vitamin enriched food shes eating. Also the kittens will bite her when they nurse as they get older causing her teats to become sore and bleed. Leaving the kittens too long together with their litter mates can cause them to become too dependent on each other causing severe anxiety for them once separated. In all my experience with cats/kittens I find it best to do the separating at 10-12 weeks before they can form too extreme bonds with each other and before mom gets sick. I would never separate before the 10-12 week frame so as the kit's are not at risk for developmental problems and the moms don't grieve unnecessarily looking for the kit's.
With all that said mom will tell you when she has had enough of her kit's. I found that this is in the 10-12th week. Rarely I've had a few that kept theirs throughout the 3rd month but never longer.